When lightning directly strikes a rail of a railroad track, very large surge currents and surge voltages are produced in the rail. Direct lightning strikes can deliver up to 200 kiloamperes of peak current. An average direct lightning strike produces peak currents of about 20 kiloamperes. Railroad signaling devices and/or other electronic equipment which are connected to the conductive rails are very likely to be seriously damaged by such surges unless adequate surge protection equipment is inserted in the electrical connections between the rails and the railroad signaling devices or the like.
So called "air gap" surge protection devices commonly are used as surge suppressors for this purpose. Conventional air gap suppressors have pointed conductive teeth that are spaced apart by a precisely determined distance. Then, if the voltage difference between the railroad rails exceeds the threshold voltage of the air gap device, the gap between the pointed teeth becomes ionized at about 1200 to 1500 volts and arcs and then produces a very low impedance short circuit between the teeth, and the surge current associated with the surge voltage is discharged through the pointed teeth and across the ionized air gap instead of into the railroad signaling equipment and thereby causing damage. A problem with such prior air gap surge suppressors is that the high surge currents tend to melt the pointed teeth, thereby both increasing the air gap spacing between the points of the teeth and rounding the points of the teeth. These effects substantially increase the threshold of the air gap suppressor, thereby increasing the amount of surge voltage that must appear between the rails, and hence across the air gap device, before the railroad signaling circuitry will be effectively protected by ionization and short circuiting of the air gap device. Such increased, unshunted surge voltage increases the likelihood and severity of damage to the railroad signaling equipment. For example, a typical prior air gap surge suppressor may require replacement after suppressing surge voltages from only about 5 to 10 average lightning strikes
Furthermore, the build-up of 1200-1500 volts between the rails may damage the signaling equipment even before a conventional air gap surge suppressor begins to protect the equipment.
Accordingly, it is an object of the invention to provide a surge suppressor that provides adequate surge suppression between a rail struck by lightning and railroad signaling equipment coupled to the rail, even though the threshold of an air gap device in the surge suppressor has increased due to melting and rounding of its pointed teeth by surges produced by prior lightning strikes.
It is another object of the invention to provide a surge suppressor that needs replacement less often than prior surge suppressors.
It is another object of the invention to provide a surge suppressor that provides a relatively constant voltage clamping level despite increases of threshold of an air gap device therein due to cumulative effects of prior lightning strikes.
Briefly described and in accordance with one embodiment thereof, the invention provides a voltage surge suppressor including a first air gap suppressor connected between a first conductor and a ground conductor, a second air gap suppressor connected between a second conductor and the ground conductor, a first air gap equalizer connected between the first and second conductors, a first inductor coupled between the first conductor and a third conductor, a second inductor coupled between the second conductor and a fourth conductor, a first surge suppressor diode having its anode connected to the third conductor and its cathode connected to the fourth conductor, a second surge suppressor diode having its cathode connected to the third conductor and its anode connected to the fourth conductor, the third and fourth conductors being adapted to supply power to a railroad signaling circuit.
In one embodiment, a third inductor is coupled between the third conductor and the first inductor, a fourth inductor is coupled between the fourth conductor and the second inductor. A second air gap equalizer is connected between the junction joining the first and third inductors and the junction joining the second and fourth inductors.